Diesel engines have higher combustion efficiency than commonly used gasoline engines and are therefore presumed to be used continuously for applications that require efficient use of fuel. However, exhaust gas emitted from diesel engines contains nitrogen oxides (hereinafter may be referred to as “NOx”), hydrocarbons (hereinafter may be referred to as “HC”), carbon monoxide (hereinafter may be referred to as “CO”), and soot composed mainly of carbon (particulate matter, which may be referred to as PM). Therefore, from the environmental point of view, it is not preferable to emit the waste exhaust gas without any treatment.
Particularly, as for automobiles, laws or regulations concerning special measures for total emission reduction of nitrogen oxides, carbon monoxide, and particulate matter from automobiles in specified areas have been enforced. Therefore, it is necessary to continuously seek methods for reducing such emission materials.
In a common exhaust gas purification method, a diesel particulate filter (DPF) composed mainly of a ceramic is used to remove PM in the exhaust gas therefrom. However, since the removed PM is accumulated in the DPF, the accumulated PM must be removed regularly. To remove the PM, heat with a temperature higher than the combustion temperature of the PM is added to the DPF to gasify the PM. Therefore, additional fuel is required for the heating, causing deterioration of fuel consumption.
If the heating temperature can be reduced, the fuel consumption will be improved. Therefore, many studies have been conducted to reduce the heating temperature. The present applicant has conducted extensive studies on catalysts, so-called oxidation catalysts, that allow PM combustion at low temperature (such activity is hereinafter referred to as “PM combustion activity”). Such catalysts have been disclosed in, for example, Patent Literature 1.
Recently, catalysts for exhaust gas purification are expected to allow not only purification of soot but also removal of NOx, HC, and CO in the gas. To remove such components, it is effective to use platinum group elements that have conventionally been considered as three-way catalysts for exhaust gas from gasoline engines. However, it is known that the purification performance of these platinum group elements deteriorates as their grains grow. Therefore, methods for suppressing the growth of the grains of the platinum group elements have been studied (see, for example, Patent Literatures 2 and 3).